Method for transporting and degasifying a melt



2 Sheets-Sheet 1 mvENToR ER\K ALLAN OLSSON Bv Qua/@41 THU E. A. OLSSONMETHOD FOR TRANSPORTING AND DEGASIFYING A MELT April zo, 1965 Filed Aug.8, 1962 ATTORNEYS April 20, 1965 E. A. oLssON 3,179,512

METHOD FOR TRANSPORTING AND DEGASIFYING A MELT Filed Aug. 8, 1962 2Sheets-Sheet 2 Fig.2

|NvENToR ERM ALLAN OLSSON BY www ATTORNEYS United States Patent O zclaims. (ci. rs-io) It has been known for some time that by degasifyingmolten metal with vacuum treatment appreciable improvements in qualitycan be obtained. Vacuum treatment of iron and steel is rapidly gainingimportance, and several methods are known and are in practical use. A.normal procedure involves placing the melting furnace or casting ladleand the mould, wherein the contents of the furnace or ladle are to becast, within an airtight container which is evacuated. Then the vacuumchamber containing the complete furnace and die is tilted so that themelt runs from the furnace down into the mould. This method is suitablemainly for a relatively small charge quantities, and then only forcasting a single ingot or a small number of ingots. For larger chargesto be cast into a number of ingots it is preferable to place the ladiesintended for casting within an evacuated container or chamber which hasbeen provided at the top with arrangements for introducing the melt froma furnace or another ladle. The main part of the charge is degasied whenthe melt enters the airless space from above as a jet. The jet breaks upunder the action of the overpressure oi the dissolved gases. Thedegasitied melt collected in this way in the ladle in the vacuum chamberis later cast into casting moulds or ingot moulds outside of thechamber. To prevent the melt from absorbing oxygen and/or hydrogenduring casting, the casting jet is enveloped by inert gas which is alsoemployed to lill the mould.

Another process is characterized by the melt in the casting ladle beingdrawn up one or more times into an evacuated receptacle so that thecontents of the casting ladle are degasitied as completely as possible.

A third process is as follows: the melt is caused to circulate from thecasting ladle or corresponding container through an evacuated chamberconnected by two pipes or channels with the ladle or container which isat a lower level. The melt is thus drawn by vacuum into the uppercontainer, and circulation of the melt is effected by blowing inert gasinto one of the pipes. After being treated according to requirements theentire contents o the ladle are cast into ingot moulds or castingmoulds.

Vacuum treatment according to the above-mentioned methods takes arelatively long time during which the temperature of the melt dropsowing to heat losses which normally occur through radiation andconvection. Other reasons for a temperature drop in the melt are theendothermic reaction which takes place with the release of gases fromthe melt, and the heat transfer to the often only slightly preheatedlinings in the vacuum treatment installations. it is therefore desirableto carry out degasiiying as quickly as possible. This requires largecontainers and large vacuum pumps.

This invention provides a method for degasifying the y li, l W l ZPatented Apr. 20, i965 ICC melt during the' casting process and permitsdegasifying of even larve charges with relatively small degasifyingequipment. The method according to the invention is characterized bypassing the melt as it is cast, through an evacuated container, whereinit is supplied with heat energy whereby temperature drop of the melt isprevented or held within permissible limits.

The invention is described more in detail in connection withinstallation for continuous casting.

FlG. l shows a rst embodiment in cross section in which only thecomponents of the installation necessary to clarity the invention areincluded.

FIG. 2 shows in a similar manner a modilied embodiment.

'he installation as shown in FIG. l comprises an airtight container 3lined with refractory material and tted with two downward pipes l and 5.The container is connected through a pipe lil with a vacuum pump (notshown) for evacuation. When the melt lills the receptacle 2. and themould 6 so that air is prevented from being drawn in through pipes 4 and5, the container 3 is iilled under vacuum with melt. The level of meltis such that the prevailing internal pressure in the container 3 plusthe weight of the liquid column over the surface of the melt in thereceptacle Z and in the mould d is balanced with the prevailing externalatmospheric pressure. When this balance is obtained and melt is suppliedto the receptacle 2 from a casting ladle l with bottom emptying and thelevel in the receptacle 2. tends: to rise, the melt runs over into themould 6 via pipe 4, container 3 and pipe :'i. The same thing happenswhen the level in the mould 6 sinks for certain reasons, eg., extractionof the partially solidilied cast 7 in the die. A means of transport forthe melt is obtained as a result of both free surfaces of the meltseeking to take up the same level. The supply of melt to the mould canbe determined by the extraction speed of the cast '7 out of the mould,assuming that the surface of the melt in the receptacle 2 is maintainedat an unchanged level by the supply of new melt regulated automaticallyin a simple manner. As the continuously moving melt to the container 3releases dissolved gasses, owing to the lower partial pressure ofrespective gases within the container 3, the vacuum pumps must be keptin continual operation. The pumps must be designed to maintain therequired reduced pressure in the chamber 3 notwithstanding the releaseof gases described above.

An improvement in degasifying can be brought about by connecting up aninletfor inert gases under the Surface of the melt so that inert gas canbubble up through the melt and generate bubbles within the melt. Thesebubbles provide a relatively large free surface, and the dissolved gasescan pass into the bubble space which initially has a zero partialpressure in relation to the dissolved gases. The enlargement of the freesurface of the melt resulting from the permeation of inert gas throughthe melt (boiling) also promotes release of gases in the melt to thespace above which is under considerably reduced pressure.

The process in question incurs relatively large heat losses due to heatbeing conducted away through container 3 and the walls of pipes d and 5and due to the endothermic reaction in connection with gas release.

3 When sms` l quantities of melt per unit time flow through thedegasiying and transnorting arrangement shown, there is a risk or themelt solidifying especially in the relatively slender pipe 5 to themould. rThis disadvantage can, however, be completely eliminated by theuse of the wellknown inductive heating method. Inserted in a spaceextending through the container 3 is a yoke 9 made from transformerplates forming the primary winding il that induces a secondary voltagein the vmelt in container 31. The melt acts as the secondary winding. lnorder to close the secondary circuit an electrically conducting rod orplate lll is'inserted to connect the inlet and outlet sides of the melt.Through a connection between the die 6 or cast 'l' and the melt inreceptacle 2, another close electrical circuit can be obtained. lt isdiilicult to maintain a high electrical eticiency with this system andtheretore a relatively low power factor must be ticipated. Instead ofthe above method it can occasionally be simpler to supply heat energythrough direct resistance heating of the melt in which case an electriccurrent may be passed directly through the melt in pipes or 5. Avertical hole may be made in the lining or" the container 3 between thetwo pipes and 5 and a water-cooled electrode inserted. The design of theelectrode should be such that melt on entering the space above theelectrode is caused to solidify close to the electrode owing to thecooling effect ot the latter, while on the other hand the metal aboveremains iolten. When voltage is applied between the electrode inquestion and the cast, an electric current will pass through and heatthe melt between these terminals. 'l he strength of this current isdetermined by the electrical resistance and the applied voltage.Similarly, an electric current can be passed through to heat the melt inthc other pipe, if, for instance, the melt in container 3 is connectedto the other terminal in the electrical circuit by an immersedelectrode. Overheating of the vacuum chamber and pipes by secondaryinduction currents may be avoided 'oy employing suitable design andmaterial.

The installation described here is well suited as a conveyingarrangement for multiple casting when it is provided with a numberof'pipe 5 correspoi rig to the num- Special regulation of the flow berof casts and moulds. of melt to each separate mould is completelyunnecessary, as each mould is automatically filled with melt to a levelcorresponding to the level of molten metal in reeptacle 2. Thisarrangement has advantages over other methods when casting from arelatively large ladle into a large number of moulds. The treatment timefor the contents of a casting ladle can be extended over the entirecasting period, consequently reducing the requirements regarding sizepumping capacity of the installation. The arrangement is provided with aiiow outlet (nozzle) corresponding to the desired casting speed (risingspeed of melt in the mould). When one mould is filled, filling istransferred to the next mould. Either the moulds are placed on cars andthe degasifying installation is stationary or vice versa. To prevent themelt from rcabsorbing undesirable gases, eg., oX gen and/ or hydrogen,during the passage into the mould, it is advantageous to fill the mouldswith inert gas and to envelop the freefalling jet of melt in inert gassupplied through nozzles arranged around the outlet opening.

irrespective of whether the electric current is supplied directly to themelt via contact devices or is induced in the melt, inwardly directedforces arise perpendicular to the direction of the current, i.e., thepinch effect. This effect serves to constrict the melt in the channelsand thus aiects the how-through of melt according to the currentdensity. Since the reduced pressure prevailing in the evacuatedcontainer has considerably reduced the static pressure of the melt thatacts against these forces, constriction is more noticeable at relativelylow current strength than would be the case if there were full staticpressure. When the current is increased to provide more heat at lowerthrough-flow in order to compensate for heat losses and temperature dropofthe melt, the constricting effect is also increased and can be usedfor regulating the rate of flow of the melt through the installation.This type of regulation has an especially important application in thecontinuous casting of ingots so small that the discharge pipe, even withthe thinnest permissible wall thickness, cannot enter the mould opening.It is a well-lrnown fact that discharge of small quantities or metalsthat are difficult to melt is combined with great dihiculties due to therisk of solidication in the nozzle.

PEG. 2 presents an arrangement that is specially suited for this case.The current is supplied to the melt directly via two electrodes la andl5, eg., graphite electrodes, which form pipes similar to l and 5 ofFIG. 1. The current provides'heat both to the melt in the evacuatedcontainer 3 `and especially to the melt in the discharge pipe where theconducting area is relatively small. Furthermore, the current tends tohave a constricting effect on the melt especially in channel il?. owingto the abovementioned pinch etect. increased current strength producesboth the constricting effect, Le., the tendency to diminish thethrough-flow of the melt, and the generation of heat in the melt. Anozzle i3 of heat resistive material is inserted in the bottoni or thepipe l5 which forms one of the electrodes. The diameter of this nozzleis suited to pass a smooth iet of melt and prevents the iet fromspreading out and deviating on the cessation of the influerce of theabove-mentioned pinch eliect. it is of course possible to supply thecurrent through an electrode in thereceptacle 2 instead ot through pipe14 as shown. It is also possible to supply the current via the cast,eg., via the mould 6 or via the rollers beneath the die which pressagainst the cast 7. However, local overheating can easily occur at theplace of contact due to Contact resistance and damage the cast.

It the above-mentioned pinch effect is not required, it may be reducedor eliminated by means of special arrangements, e.g., by arrangingparallel conductors with opposite current directions. Both conductorsmay consist of the melt, or only one may consist or the melt, while theother comprises another conductor with a larger area and/ or a largerconducting capacity than the column o melt.

I claim:

l. A method vfor continuously transporting and degasitying molten metalduring continuous casting, comprising the steps of continuously feedingmolten metal t0 a container subject to atmospheric pressure,continuously evacuating and drawing orf gases in a degassing chamberisposed above said container, continuously siphoning molten metal fromsaid container through a first conduit into and through said degassingchamber and discharging the molten metal to a continuous casting moldthrough a second conduit connected to the bottom of the degassingchamber, heating the molten metal while passing through the degassingchamber by passing electric current directly through the moving moltenmetal in said tirst and second conduits and said degassing chamber, andincreasing and decreasing the flow of molten metal siphoned from saidcontainer through the degassing chamber to said mold by respectivelydecreasing Yand increasing the electric current supplied for heatingthereby decreasing and increasing, respectively, the constrictinginfluence of the pinch elect in said conduits while preventingsoliditication of the molten metal flowing in said conduits.

2. A method for continuously transporting and degasitying molten metalduring continuous casting, comprising the steps of feeding molten metalto a container subject to atmospheric pressure, evacuating and drawingoff gases in a degassing chamber disposed above said container,siphoning molten metal from said container through a first conduit intoand through said degassing chamber and discharging the molten metal to acontinuous casting mold through a second conduit connected to the bottomof the degassing chamber, inductively heating the molten metal whilepassing through the degassing chamber by passing electric currentdirectly through the moving molten metal in said first and secondconduits and said degassing chamber, and increasing and decreasing theow of molten metal sphoned from said container through the degassingchamber to said mold by respectively decreasing and increasing theelectric current supplied for heating thereby `decreasing andincreasing, respectively, the constricting iniluence of the pinch effectin said conduits while pre- References Cited by the Examiner UNlTEDSTATES PATENTS Rozian 75-49 Harders et al 2654-34 Lorenz 75-93 X Lorenz75-93 Chambers 22-73 LorenzV 75-49 venting solidication of the moltenmetal flowing in said 10 DAVID L RECK Primary Examiner conduits.

1. A METHOD FOR CONTINUOUSLY TRANSPORTING AND DEGASIFYING MOLTEN METALDURING CONTINUOUS CASTING, COMPRISING THE STEPS OF CONTINUOUSLY FEEDINGMOLTEN METAL TO A CONTAINER SUBJECT TO ATMOSPHERIC PRESSURE,CONTINUOUSLY EVACUATING AND DRAWING OFF GASES IN A DEGASSING CHAMBERDISPOSED ABOVE SAID CONTAINER, CONTINUOUSLY SIPHONING MOLTEN METAL FROMSAID CONTAINER THROUGH A FIRST CONDIUT INTO AND THROUGH SAID DEGASSINGCHAMBER AND DISCHARGING THE MOLTEN METAL TO A CONTINUOUS CASTING MOLDTHROUGH A SECOND CONDIUT CONNECTED TO THE BOTTOM OF THE DEGASSINGCHAMBER, HEATING THE MOLTEN METAL WHILE PASSING THROUGH THE DEGASSINGCHAMBER BY PASSING ELECTRIC CURRENT DIRECTLY THROUGH THE MOVING MOLTENMETAL IN SAID FIRST AND SECOND CONDIUTS AND SAID DEGASSING CHAMBER, ANDINCREASING AND DECREASING THE FLOW OF MOLTEN METAL SIPHONED FROM SAIDCONTAINER THROUGH THE DEGASSING CHAMBER TO SAID MOLD BY RESPECTIVELYDECREASING AND INCREASING THE ELECTRIC CURRENT SUPPLIED FOR HEATINGTHEREBY DECREASING AND INCREASING, RESPECTIVELY, THE CONSTRICTINGINFLUENCE OF THE PINCH EFFECT IN SAID CONDIUTS WHILE PREVENTINGSOLIDIFICATION OF THE MOLTEN METAL FLOWING IN SAID CONDIUTS.